There are two major objectives of the proposed investigation. One is to determine principles governing virus morphogenesis. The principal lines of this investigation are: 1) How do the viral-core components -Internal-proteins - of a virus nucleate head formation? 2) Is final head shape determined to any extent by the structure formed by the core proteins? 3) Is DNA a part of this assembly core structure, and can DNA virus structural protein precursors to the head be isolated? 4) How does DNA condensation occur and what is the structure of DNA in the mature phage head? Is the binding of internal proteins to DNA involved in DNA condensation, and are these binding properties altered by proteolytic cleavage of the internal proteins during assembly? The second major objective of the proposed investigation is to determine the mechanism of virus initiation of host cell infection. How is cell metabolism taken over during virus infection and what steps initiate the process? How does an internal protein of phage T4 act to allow certain types of E. coli to be successfully infected? By what mechanism does the injected internal protein increase the probability of host range of the virus?